Wednesday, January 29, 2014

Doing my bit to encourage warm thoughts for those of us in the frozen north...

I present to you an island off the coast of what is now California, approximately 15,000 years ago. This island is called Santarosae (or Santa Rosae); as sea level rises following the melting of the great continental ice sheets, it will be dissected into the four northern Channel Islands (San Miguel, Santa Rosa, Santa Cruz, and Anacapa), which are eventually reunited as Channel Islands National Park (with Santa Barbara Island). These events, though, are in the future. For "now", we have one large island a handful of miles from the coast:

A. The modern northern Channel Islands. B. The geography of the islands at various times in the recent past. From Collins (2009).

Saturday, January 25, 2014

I'd like to start off with a couple of photographs, to illustrate some
facts of life concerning our old friend, the Platteville Formation:

An ever-so-slightly hazardous ledge along the road into Crosby Farm
Regional Park.

Hidden Falls Regional Park: when good rocks go bad.

The Platteville Formation just loves to ledge out, particularly the lower part, which rests on the recessive Glenwood Formation, and once you get heavy rocks ledged out far enough... About ten thousand years ago, not very far from the second photo, there was a ledge of the Platteville Formation left jutting out in the figurative "wake" of the erosion of the Mississippi River system. At some point, a dog-sized rodent opted to go underneath it. Maybe this was its usual shelter, or maybe it was driven there by some circumstance; we'll never know, short of time travel. Unfortunately for the rodent, the slab failed, with predictable results. It would be covered by the debris until July of 1938, when a Works Progress Administration crew uncovered the site while widening what is now the north entrance road into Hidden Falls Park. Its skeleton, crushed and coated with calcite leached out of the slab, was recovered and eventually prepared, reconstructed, put on display at the Science Museum of Minnesota, and described scientifically as an example of the extinct mammal known as Castoroides ohioensis, a.k.a. the giant beaver.

From Powell (1948): the WPA project that led to the discovery (site with the superimposed white circled cross).

"Well," you say, "that was refreshingly concise." Ah, but there's always more to say.

Wednesday, January 22, 2014

Anchisaurusis kind of a second-tier dinosaur, not up there with Tyrannosaurus, Triceratops, Velociraptor, and their ilk, but it gets a fair amount of exposure, especially if you pull in the old synonym Yaleosaurus. It gets points for having a long history, having a few interesting anecdotes associated with it, being one of the first early dinosaurs known from good remains, and beginning with the letter "A"; this latter bit is important, because it means Anchisaurus ends up near the beginning of children's A-Z dinosaur books. (Is this why so many dinosaur names begin with "A"?)

Sunday, January 19, 2014

There comes a time when we must speak of the Platteville Formation in detail. If there wasn't a Platteville Formation, there would be no narrow gorge in the Twin Cities, no Minnehaha Falls or Hidden Falls, and most importantly no Saint Anthony Falls: if there's no Saint Anthony Falls, Minneapolis develops in a radically different way. No falls, no easy water power, no mills. The Twin Cities owe their existence to a 455 million year old carbonate bank and the vagaries of glacial and river erosion.

Wednesday, January 15, 2014

I'd like to plug the Minnesota Geological Survey, in particular their fantastic collection of online publications. Everything published by the modern Survey or its 19th century incarnation is available for download as pdfs. Looking for, say, the fossil volume of The Geology of Minnesota monograph series? Parts One and Two. The geology of Anoka County? Right here. A publication on Minnesota brick and tile manufacturing? Here (and it's by a man named Grout, which just seems like some strange kind of destiny). Anyway, if you poke around the main list, found at the second link above, you'll find just about anything that can be written about Minnesota rocks. If you're not especially familiar with geology, I'd recommend starting with the "Minnesota At A Glance" documents under "Miscellaneous Publications", or the publications under "Educational Series".

I'd like to draw your attention to one of the newest publications, "Bedrock geology of the Twin Cities ten-county metropolitan area, Minnesota" by John H. Mossler. This is the newest in a line of geologic maps of the Twin Cities going back to the 19th century. The link I provided has three links at the bottom; unless you have GIS software, I'd recommend just going for the first link, "10 county map.pdf". To briefly explain if you haven't had much contact with geologic maps, this map shows the locations of various rock formations, as well as folds and faults (not that we're swarming with them in east-central Minnesota, but there are some). The rock formations are denoted by color and by a short letter code; for example, the Decorah Shale is represented by a teal color with Od, the "O" for the Ordovician Period and the "d" for the Decorah. (the map is a bit inconvenient in that several of the formations have been given similar shades of blue; fortunately, because the rocks in question are flat-lying with few folds and faults to break up the succession, it's usually not much of a problem to tell what is what once you are familiar with the order.) Older rocks are found where the younger rocks have been eroded, such as in the various modern and ancient river channels.

An important caveat is illustrated by the cross-section, which shows an otherwise un-mapped brown layer covering almost everything to the tune of a couple hundred feet (60+ m); this brown layer represents Quaternary deposits, mostly the stuff so generously left behind by glaciers. In other words, the main map is what you'd get if you vacuumed up all of the loose stuff on top. The terrain left behind would be kind of strange, because the glacial deposits cover a lot of old river channels (every time there was a glacial advance, the previous channels would be buried, and new channels would be excavated after the glaciers retreated; some of the older channels cut down significantly deeper than the modern channels – the modern Mississippi above Fort Snelling doesn't cut into "yellow" rocks [Jordan Sandstone] until it gets out of the central metro, for example). The majority of the map was made from subsurface data, such as well logs; the ubiquitous circles are water wells with logs. Areas where the bedrock is exposed at the surface (and which are large enough to be mapped at a scale of 1:125,000) are denoted by a dark gray color. Zooming down to 300x or 400x, where the street grids become comfortable to read, shows that most of these outcrops are near the modern river system. This is because the river have cut through the glacial deposits into the bedrock. Some of the large areas of dark gray pick out quarries, like the one on Grey Cloud Island. There are also thumbnail descriptions of the formations at the bottom, describing the thicknesses, what the formations are made of, and so forth.

Sunday, January 12, 2014

Clinton R. Stauffer had one of the most convenient field areas it is possible for a geologist to have. He worked for the University of Minnesota from 1914 to 1944, producing a number of papers on paleontology and stratigraphy; they include a study of the Paleozoic of Minnesota (Stauffer and Thiel 1941), a list of Pleistocene mammal finds from the state (Stauffer 1945; an update would be greatly appreciated!), and several descriptions of microfossils (Stauffer 1930, 1933, 1935a, 1935b). He did a fair amount of collecting for the university, and a fair amount of collecting within the university. "Aha," you may say if you are familiar with the geography; "the campus is split by the river, with prominent and accessible bluffs." This is quite correct, but he did not limit his on-campus collecting to the bluffs. For example, during the construction of Northrop Auditorium in 1927, he obtained rocks from the excavation for the heating shaft (Stauffer 1930), and described a number of conodonts and other fossils from this material (Stauffer 1930, 1933, 1935a). If he wanted comparable material from other locations, it was only a matter of miles to southwestern St. Paul/southeastern Minneapolis, where he worked extensively in the Ford Plant/Fort Bridge/Minnehaha Creek/Lock & Dam 1 area (this was convenient both in terms of location and time; the dam, auto plant, and bridge were all completed during this time frame, so there was a lot of disturbed ground and excavated rock to pick through). It wasn't all roses and brachiopods, though; those bluffs on campus are not a place for anyone who have a healthy respect for gravity and large heavy rocks. The combination of a narrow footpath and overhanging rock makes the area about the most hazardous I have seen for Twin Cities geology.

Wednesday, January 8, 2014

There are some things in life that just don't have a handy name. Take for example the subject of this entry, an obscure group of mollusks known from a few modern deep-water species and a more robust fossil record. Various members of this group have labored under the terms "Monoplacophora", "Tergomya", and "Tryblidiida", all of which could be mistaken for a disease, a planet in a science fiction series, or a piece of anatomy you'd rather not know about. (You can combine all three: "I got monoplacophora in my tryblidiida after I visited Tergomya. It's as bad as it sounds.") At one point someone suggested that their vernacular name should be "gastroverm", which you'll probably agree was no help at all. The reason for this multitude of names has to do with the rules of classification; visit here or here if you want details (and don't say I didn't warn you), but the basic bit is that Tryblidiida includes all the modern forms, and is classified in Tergomya, which is in turn part of Monoplacophora, and "Monoplacophora" is something of a problem because its traditional definition includes some things that weren't all that closely related. However, everyone was kind of used to Monoplacophora, so they kept it around, except now it can have a precise definition, or be a "state of mind" including various odd extinct things from the Cambrian, which I shall omit. If you want to be picky, this is about tergomyans.

Saturday, January 4, 2014

Investigations of the geology and paleontology of the Twin Cities area of Minnesota go back a surprisingly long time. Several descriptions can be found from the decades before statehood (1858), included as parts of larger surveys (i.e. Keating [1824], Featherstonhaugh [1836], Nicollet [1843], and several publications by David Dale Owen [1847, 1848, 1852]). The earliest report I am familiar with was published by William H. Keating, concerning an 1823 expedition. Keating was something of a polymath who wore a number of different hats during his relatively short life (1799–1840), including three years' work in the schools and mines of Europe (Miles 1959). In 1823 he joined an expedition under the noted explorer Major Stephen H. Long, as the title of his 1824 publication makes clear (this was back when a title was also an abstract and introduction).